It’s British and means your jaw hit the floor and stayed there because you were too astonished to close your mouth.
Reader: I was gobsmacked when I read your blog.
Yearstricken: I’m so glad. I really enjoy gobsmacking others.
Reader: Yes, there’s really nothing quite as bracing as a good gobsmack.
Yearstricken: Well, tally ho and all that, I must get back to my blog. There are still so many gobs to smack.